The present invention pertains to an electrochemical gas sensor and more particularly to an electrochemical gas sensor with a plurality of electrodes, with an electrolyte and with a gas-permeable membrane.
Such a gas sensor is shown in DE 199 39 011 C1, where the measuring electrode consists of diamond-like carbon (DLC). In this prior-art electrode, the diamond-like carbon is preferably applied to the gas-permeable membrane by a sputtering process in a radio frequency magnetron sputtering unit. This prior-art arrangement is very well suited for many applications, but the chemical stability it ensures and the potential window that can be reached are not sufficient for all measurement applications.
The object of the present invention is to propose an improved gas sensor of the type mentioned in the introduction, which has a reduced cross sensitivity with respect to interfering gases, has a short response time and high sensitivity for the measured gas without the service life being reduced. The measured gas may occur either in a mixture with other gases or even dissolved in a liquid, especially water.
According to the invention, an electrochemical gas sensor with a plurality of electrodes is provided. The sensor has an electrolyte and a gas-permeable membrane. At least the measuring electrode comprises a thin layer made of doped diamond on a porous substrate.
The thin layer may consist of the diamond doped with boron, especially with a doping corresponding to 1019 to 1021 boron atoms per cubic centimeter. The thin layer may also consist of the diamond doped with nitrogen. The doping particularly corresponds to about 1020 nitrogen atoms per cubic centimeter.
The thickness of the thin layer consisting of the doped diamond is advantageously 0.5 xcexcm to 5 xcexcm.
The thin layer consisting of the doped diamond may be advantageously prepared by deposition from the gaseous phase (CVD, Chemical Vapor Deposition). The porous substrate may be a quartz nonwoven. The layer thickness may be advantageously 0.2 xcexcm to 0.5 xcexcm. The quartz nonwoven may have a surface coating. This is preferably a silicon carbide or silicon nitride coating.
The surface of the thin layer consisting of the doped diamond is additionally doped with a precious metal, especially with gold, platinum and/or iridium. The auxiliary electrode may be designed as a thin layer consisting of the doped diamond on a porous substrate.
The essence of the present invention, that at least the measuring electrode is designed as a thin layer consisting of diamond doped with boron or nitrogen on a porous substrate has many advantages. The porous substrate preferably consists of the nonwoven material made of chemically pure quartz. The extent of the doping determines the optimal ranges for good, desirable electrical conductivity.
The preferred process for producing the diamond layer, by deposition from the gaseous phase (CVD, Chemical Vapor Deposition), is known per se from the publication xe2x80x9cSynthetic Diamond, a New Electrode Material for Electroanalysis,xe2x80x9d Yu. V. Pleskov, Journal of Analytical Chemistry, Vol. 55, No. 11, 2000, pp. 1045 to 1050 and the references cited therein. In the diamond electrodes known currently, the diamond layers are applied either to a closed, planar substrate or to a metal grid or network.
According to the present invention, the thin diamond layer is applied according to the prior-art CVD process to a porous substrate, especially a quartz nonwoven. As a result, it is possible to produce the microporous electrode structures necessary for electrochemical gas sensors with a pore size of 0.1 xcexcm to 100 xcexcm, which is permeable to the electrolyte and the gas to be measured. Another advantage of the electrode structure thus prepared with the porous substrate is the good mechanical flexibility, which prevents the undesired formation of electrolyte gaps for the diffusion membrane.
The following essential advantages are observed in the case of gas sensors according to the present invention:
Extremely low residual currents are measured, so that even very low concentration ranges can be measured,
low catalytic activity is observed, i.e., the reactions taking place at the electrode because of interfering gases, e.g., hydrogen sulfide (H2S), are strongly inhibited,
high long-term stability of the measured signal is obtained,
a wide potential window is obtained, so that a wider range of measurement can be covered with respect to different measured gases,
the double-layer capacity is low, and it also undergoes hardly any changes under temperature and moisture effects.
The widened potential window and the markedly improved chemical stability also make possible the conversion of highly chlorinated hydrocarbons, which do not react at conventional measuring electrodes of electrochemical gas sensors.
To specifically modify the catalytic properties, the surface of the electrodes according to the present invention may be doped with a precious metal. The surface of the porous substrate, especially the quartz nonwoven, can be chemically modified by applying a silicon carbide or silicon nitride layer.